The invention relates to a process for the thermal cracking of hydrocarbons for the production of olefins.
To produce olefins, it is conventional and advantageous to employ light hydrocarbons, such as, for example, ethane or propane, or hydrocarbon mixtures having a boiling point of below 200.degree. C., such as, for example, naphtha, as starting materials for a thermal cracking operation. These starting materials result in a high yield in olefins and relatively few undesirable by-products.
However, in view of the high demand for olefins, which may lead to a short supply and increase in price of the aforementioned advantageous starting materials, several attempts have been made through the years to develop processes which permit the utilization of higher-boiling starting materials. The basic problem involved in the use of higher-boiling starting materials, i.e., above 200.degree. C., is a reduced olefin yield and the formation of liquid cracked products, the proportion of which increases sharply as the boiling range of the starting material increases. The liquid cracked products are generally separated into a fraction boiling below 200.degree. C. and into a fraction boiling above 200.degree. C. The lower-boiling fraction represents a high-octane fuel and contains valuable components, such as benzene, toluene, and xylene. The fraction boiling above 200.degree. C., in contrast thereto, constitutes an undesirable product containing highly condensed aromatics, polymeric compounds, and sulfur compounds. The proportion of this fraction (called pyrolysis fuel oil hereinbelow) is, when cracking naphtha, in the range of about 1-5% by weight of the total products but rises when using gas oil to a magnitude of 30% by weight, and to still higher values when using heavier charges, such as vacuum gas oil or crude oil, or crude oil residues. The sulfur contained in the starting material is enriched in the pyrolysis fuel oil fraction in such quantities that the combustion of this fuel alone without admixing low-sulfur fuels leads to a waste gas which is unduly contaminated. The mixture with low-sulfur fuels, however, is connected with additional problems, since pyrolysis fuel oil is miscible only to a limited extent with crude oil distillates and thus can be blended with the latter only partially. Another disadvantageous property of the pyrolysis fuel oil is to be seen in that it lends itself only under certain conditions to storage and transportation.
The production of olefins by cracking hydrocarbon mixtures having a boiling range of above 200.degree. C., such as, for example, gas oil or vacuum gas oil is economically infeasible, unless measures are taken to reduce the quantities of thus-formed pyrolysis fuel oil or unless the pyrolysis fuel oil can be passed on to some other economical usage.
A process has been known from DOS (German Unexamined Laid-Open Application) No. 2,164,951 (equivalent to U.S. Pat. No. 3,781,195) which is suitable for the production of olefins from high-boiling hydrocarbon mixtures. In this process, the starting material is catalytically hydrogenated, prior to the thermal cracking thereof, in the presence of hydrogen. The hydrogenating pretreatment leads to a reduction in the content of polyaromatic compounds which are essentially responsible for the formation of the pyrolysis fuel oil. Moreover, a desulfuration of the starting material takes place as well. The operation of such a process, however is burdened by high initial investment costs for a plant for the hydrogenation of the starting material and by high operating costs since large amounts of hydrogen must be made available.